There has been a wavelength conversion laser device that performs wavelength conversion using a nonlinear optical phenomenon of a wavelength converter, so that the wavelength of a fundamental wave laser beam is converted to a converted wave laser beam, such as second harmonic generation (SHG), sum frequency generation, and difference frequency generation.
As shown in FIG. 27, for example, a wavelength conversion laser device is made up of a fundamental laser light source 101, a lens 102 that condenses a fundamental laser beam emitted from the fundamental laser light source 101, a wavelength converter 103 that generates a second harmonic wave of the condensed fundamental laser beam, and a dichroic mirror 104 which separates a fundamental laser beam from a harmonic laser beam.
The wavelength converter 103 is made of a nonlinear optical crystal and performs wavelength conversion of a fundamental wave by appropriately adjusting the orientation and the poled structure of the crystal and the like so that the fundamental wave and the converted wave are phase-matched. In particular, a wavelength converter using the poled structure is capable of performing wavelength conversion at high efficiency even at low power by pseudo phase matching, and is thereby capable of performing wavelength conversion in various manners according to designs. The poled structure referred to herein is a structure having a region in which the spontaneous polarization of the nonlinear optical crystal 103 is inverted periodically.
The conversion efficiency η for converting a fundamental wave into a second harmonic wave is expressed as:η∝L2P/A×sin c2(ΔkL/2)where L is the interaction length of the wavelength converter, P is the power of the fundamental wave, A is the beam sectional area at the wavelength converter, and Δk is a shift from the phase matching condition. Under the suitable condensing condition for the interaction length L, the expression is rewritten as:η∝LP×sin c2(ΔkL/2).
The conversion efficiency can be increased by making the interaction length L longer. However, because tolerance ranges for a shift from the phase matching condition are inversely proportional to L, a problem arises in that strict conditions are required for the adjustment and the fundamental waves. In particular, with a high-output wavelength conversion laser device, the wavelength converter generates heat as the wavelength converter absorbs the fundamental wave and the converted wave. This heat generation makes the temperature of the wavelength converter inhomogeneous, which would result in lower conversion efficiency. This phenomenon is noticeable particularly for high intensity wavelength-converted laser beams.
Various proposals have been made to date in order to realize an improved conversion efficiency of the wavelength conversion laser device. For example, JP-A-11-44897 proposes the arrangement which increases the conversion efficiency with the use of a plurality of wavelength converters and light-collecting means. Also, JP-A-2006-208629 proposes a configuration to provide the wavelength converter with a reflector for a fundamental laser beam so that the fundamental laser beam re-enters into the wavelength converter. In addition, JP-A-2005-268780 proposes a configuration to dispose a wavelength converter between opposing reflection mirrors so as to perform wavelength conversion of a reciprocating fundamental laser beam. Further, JP-A-5-265058 proposes a configuration to inject a fundamental laser beam into a resonator, so that wavelength conversion is performed by condensing the fundamental laser beam to the optical axis of the resonator.
With the foregoing conventional structures, the conversion efficiency of the wavelength conversion laser device can be improved. However, they fail to find ways to realize wider tolerance ranges in view of a shift from the phase matching condition while maintaining high conversion efficiency. With wider tolerance ranges for a shift from the phase matching condition, it becomes possible to enhance the stability and the reliability of the device, which in turn make the device compact. Furthermore, when the power of the fundamental laser beam is concentrated to particular positions in the nonlinear optical crystal, the wavelength conversion efficiencies by the nonlinear optical crystal would be lowered due to light-induced damage and heat generation, resulting in unstable operations of the wavelength conversion laser device.
The foregoing conventional structures realize higher frequency conversion efficiencies when the wavelength conversion laser device outputs low power frequency converted laser beams. However, they fail to consider heat generation for high power frequency converted laser beams, and therefore such problems as lower conversion efficiencies and complex temperature controls for high power converted laser beams remain unsolved. The foregoing conventional structures also fail to wind ways to realize higher conversion efficiencies when adopting a multi-mode laser light source for the fundamental laser light source.
Furthermore, conventional wavelength conversion laser devices oscillate only a laser beam having a narrow spectrum width due to strict phase matching conditions, and large interference noises by laser beams raise additional problems in the field of video or the like.